In a prior art ultrasonic diagnostic apparatus capable of depicting movement of a moving part within a body to be examined in the form of a differential image, as described in JP-A-62-189054, it has been proposed that an echo signal coming from an ultrasonic wave transmitting-receiving section is digitized and there are disposed a plurality of frame memories, in which data of tomographic images are stored sequentially, each of them storing data for one image, in a digital scan converter, which writes and reads the data in and from the frame memories contained therein and at the same time there is disposed on the output side of these frame memories a differential processor, in which the sequential data outputted from the different frame memories are inputted and which effects differential processing therebetween.
Further, as described in Report of 29th Meeting of the ME Society of Japan "3-H-3 Development of Digital Subtraction Echography (Medical Electronics and Living Body Technology, Vol. 28, p. 494, special print (1990) (in Japanese)), it has been proposed that an electrocardiogram of the body to be examined described above is displayed together with the differential image of the moving part in the body to be examined and at the same time a timing mark consisting of white and black parallel bars indicating the time difference between two images, for which a differential operation has been effected, is displayed.
However, in the first prior out example, although it is possible to extract only moving components for the moving part in the body to be examined to display them by using a differential image, no time difference between the two images, for which the differential operation has been effected to obtain the differential image, is displayed. Consequently the amount of displacement of the moving part described above can be detected, but the time required for the displacement cannot be obtained and therefore the speed of the displacement cannot be estimated. For this reason, diagnosis cannot be effected for the moving part in the body to be examined.
Further, in the second prior art example described above although the timing mark indicating the time difference between the two images, for which the differential operation has been effected, can be displayed on the electrocardiogram together with the differential image for the moving part in the body to be examined, this timing mark displays only indirectly the time difference corresponding to the differential operation by using the magnitude of the interval between the two parallel bars. Therefore it was impossible to display directly the time difference in the form of a numerical value. Consequently it was impossible to obtain directly the time required for the displacement of the moving part in the body to be examined and it happened that estimation of the speed of the displacement cannot be effected easily. For this reason, also in the second prior art example, diagnosis cannot be effected precisely for the moving part in the body to be examined.